An optical fiber (i.e., a glass fiber typically surrounded by one or more coating layers) conventionally includes an optical fiber core, which transmits and/or amplifies an optical signal, and an optical cladding, which confines the optical signal within the core. Accordingly, the refractive index of the core nc is typically greater than the refractive index of the optical cladding ng (i.e., nc>ng).
Multimode optical fibers are commonly used for short-distance applications requiring a broad bandwidth, such as local networks or LAN (local area network). The core of a multimode optical fiber typically has a diameter of between about 50 microns and 62.5 microns, whereas the core of a single-mode optical fiber typically has a diameter of between about 6 microns and 9 microns. In a multimode optical fiber, for a given wavelength, several optical modes are propagated simultaneously along the optical fiber.
Multimode optical fibers have been the subject of international standardization under the ITU-T G.651.1 recommendations, which, in particular, define criteria (e.g., bandwidth, numerical aperture, and core diameter) that relate to the requirements for optical fiber compatibility. The ITU-T G.651.1 standard (July 2007) is hereby incorporated by reference in its entirety.
The numerical aperture (NA) of an optical fiber can be approximated by the following equation:NA=√{square root over (nc2−ng2)}where nc is the refractive index of the central core and ng is the refractive index of the outer cladding (e.g., an outer optical cladding).
For optical fibers, the refractive index profile is generally classified according to the graphical appearance of the function that associates the refractive index with the radius of the optical fiber. Conventionally, the distance r to the center of the optical fiber is shown on the x-axis, and the difference between the refractive index (at radius r) and the refractive index of the optical fiber's outer cladding (e.g., an outer optical cladding) is shown on the y-axis. The refractive index profile is referred to as a “step” profile, “trapezoidal” profile, “parabolic” profile, or “triangular” profile for graphs having the respective shapes of a step, a trapezoid, a parabola, or a triangle. These curves are generally representative of the optical fiber's theoretical or set profile. Constraints in the manufacture of the optical fiber, however, may result in a slightly different actual profile.
For the same propagation medium (i.e., in a step-index multimode optical fiber), the different modes have different group delay times. This difference in group delay times results in a time lag (i.e., a delay) between the pulses propagating along different radial offsets of the optical fiber. This delay causes a broadening of the resulting light pulse. Broadening of the light pulse increases the risk of the pulse being superimposed onto a trailing pulse, which reduces the bandwidth (i.e., data rate) supported by the optical fiber. The bandwidth, therefore, is linked to the group delay time of the optical modes propagating in the multimode core of the optical fiber. Thus, to guarantee a broad bandwidth, it is desirable for the group delay times of all the modes to be identical. Stated differently, the intermodal dispersion should be zero, or at least minimized, for a given wavelength.
To reduce intermodal dispersion, the multimode optical fibers used in telecommunications generally have a core with a refractive index that decreases progressively from the center of the optical fiber to its interface with a cladding (i.e., an “alpha” core profile). Such an optical fiber has been used for a number of years, and its characteristics have been described in “Multimode Theory of Graded-Core Fibers” by D. Gloge et al., Bell system Technical Journal 1973, pp. 1563-1578, and summarized in “Comprehensive Theory of Dispersion in Graded-Index Optical Fibers” by G. Yabre, Journal of Lightwave Technology, February 2000, Vol. 18, No. 2, pp. 166-177. Each of the above-referenced articles is hereby incorporated by reference in its entirety.
A graded-index profile (i.e., an alpha-index profile) can be described by a relationship between the refractive index value n and the distance r from the center of the optical fiber according to the following equation:
                    n        =                              n            1                    ⁢                                    1              -                              2                ⁢                                                      Δ                    ⁡                                          (                                              r                        a                                            )                                                        α                                                                                        (                  Equation          ⁢                                          ⁢          1                )            wherein,
α≧1, and α is a non-dimensional parameter that is indicative of the shape of the index profile;
n1 is the maximum refractive index of the optical fiber's core;
a is the radius of the optical fiber's core; and
                    Δ        =                              (                                          n                1                2                            -                              n                0                2                                      )                                2            ⁢                          n              1              2                                                          (                  Equation          ⁢                                          ⁢          2                )            
where n0 is the minimum refractive index of the multimode core, which may correspond to the refractive index of the outer cladding (most often made of silica).
A multimode optical fiber with a graded index (i.e., an alpha profile) therefore has a core profile with a rotational symmetry such that along any radial direction of the optical fiber the value of the refractive index decreases continuously from the center of the optical fiber's core to its periphery. When a multimode light signal propagates in such a graded-index core, the different optical modes experience differing propagation mediums (i.e., because of the varying refractive indices). This, in turn, affects the propagation speed of each optical mode differently. Thus, by adjusting the value of the parameter α, it is possible to obtain a group delay time that is virtually equal for all of the modes. Stated differently, the refractive index profile can be modified to reduce or even eliminate intermodal dispersion.
Typically, multimode optical fibers with higher numerical apertures have lower macrobending losses (referred to hereafter as “bending losses”).
Conventional multimode optical fibers having a central core diameter of more than 50 microns are generally expected to provide sufficient bend resistance for many applications. Such exemplary optical fibers may have a central core diameter of 62.5 microns and a numerical aperture of 0.275 or a central core diameter of 80 microns and a numerical aperture of 0.3.
Nevertheless, for tighter bend radii (e.g., 5 millimeters), such optical fibers exhibit significant bending losses that may be critical for high speed transmission (e.g., in compact consumer electronic devices).
International Publication No. WO 2010/036684, which is hereby incorporated by reference in its entirety, deals with large-core optical fibers. The disclosed optical fibers, however, have a central-core radius a and a relative refractive index difference Δ such that:
                    2        ⁢        Δ              a    <      5.1    ×          10              -        3              ⁢                  ⁢                  μm                  -          1                    .      
The central cores of the disclosed optical fibers fail to provide reduced microbending losses, because, for a given Δ value, enlarging the central core will result in larger microbending losses. Furthermore, the relationships between the disclosed central-core radius a and the relative refractive index difference Δ lead to undesirably large microbending losses.
Therefore, a need exists for a multimode optical fiber having reduced bending losses and a central-core diameter of greater than 50 microns.